Transmission control in two-way signaling systems



Feb. 24, 1942. L. G. ABRAHAM 2,274,392

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS Filed April 18, 1941Fla.

E CHO SUPPRESSOR /E$ T A f S/ ,73 AMP DET s 7 19 I2 I I3 (I! AMP oar a AAMP 4I m/vawroe L. G. ABRAHAM ATTORNEY Patented Feb. 24, 1942TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS Leonard. G. Abraham,Madison, N.. .L, assignor to Bell Telephone Laboratories, Incorporated,New York; N. Y., a corporation" of New York ApplicationAprilxIS, 1941*,SeriaLNo. 389,160

6- Claims.

The: invention. relates to twoeway signal transmission systems andparticularly: to circuits for controlling" transmissiom in such systems.

Proper operation of long two-way telephone systemshas required the useof voice-operated.

switching devices, commonly known as echo suppressors, to directionallycontrol signaltransmission in: the oppositely. directed repeating pathsof the system while suppressing echoes and preventing singing. Such echosuppressors may comprise, for example, an amplifier-rectifier-relaycircuit connected to-each repeating path,.operating inresponse'tos'ignal transmission-in that path todisable theoppositelydirect'ed' repeating pathto suppress the signal echoes;

Such echo. suppressorsoften introduce detrimental effects ontransmission, such as an increase in the time required for a listeningsubscribertobreal in. on a talking subscriber, and

partial or complete signal transmission lockout When-both subscribers atthe two ends-of the system start talking atab'out' the same time. Thereare many calls in present-day systems which do not require the operationof echa suppressors to suppress echoes but which causetheecho'suppressors, which are present because they willbe needed on othercalls involving the same circuit, to function, and there is atv presentno means available for distinguishing adequately whichca-ll's doneed-echo supressors;

It is an object of the invention to'red'uce the efi ects of echo'es in"atwo way telephone system while" preventing or reducing theabove-mentioned detrimental effects;

This object is attainedin accordance with the invention by circuitsoperating automaticallyto remove an echo suppressor from a system or toefiectively' disabl'euit when it is not needed to suppress echoes. In.oneembodiment, these circuits compare the voice. currentsreceived from atwoway line on.one side-of afour-wire telephone circuit withthe. echoes.sent back in. the other. directionfrom the two-wayline on. the otherside of the, four-wire circuit, and if and when the echoes returned areweaker than the direct speech currents byanamountpredetermined assatisfactory from an echo standpoint, remove or disable the echosuppressor and restoreit when the echoes become relatively stronger thanthe limiting amount.

Th invention will be better understood from thefollowing:detaileddescription when read in conjunction with the accompanyingdrawing, Figs. 1 andZ of which show schematically a portion of a two-waytelephone system embodying diflerent modifications of the invention.

Fig. 1 shows a four-wire terminating circuit at the east terminal of atwo-way telephone system. It comprises a; westto-east incoming two-wirepathtEA including the one-way amplifying device Ala-and an: outgoingeast-to-west two-wire-circuit WA including the one-way amplifying deviceAw; coupled by th usual three-winding or twocoil hybrid transformer H inconjugate relation with each:ot'her and in energy-transmitting relationwith the two-way telephone circuit TCwhich may lead to an eastsubscribers telephone set (not shown); The circuits EA and WA may extendto the west to a similar four-Wire terminating circuit at the westterminal of the system directly' or through an intermediate radio ortwo-way wire. link (not shown);

An echo suppressor represented by the box ES, which may be of any of theWell-known types, is associated with the paths EA and WA at the eastterminal, as indicated. In the particular type of echo suppressordiagrammatically illustrated, the incoming path EA is normally enableddue to the normal break in the short-circuiting connection across thatpath at the point I; provided by the normal open condition of thecontacts of the unenergized switching relay RE, and the outgoing path WAis normally enabled due to the normal break in the short-circuitingconnection across the latter path at the point 2, provided by the normalopen condition of the contacts of the unenergized switching relay Rw. Inthe ordinary operation of the echo suppressor ES, a portion of theincoming telephone signals received over the path EA from a westsubscriber, diverted into the transmitting switching control TS, whichmayinclude an amplifierrectifler, not shown, will cause the operation ofthe relay Rw to close its switching contacts to short-circuit the pathWA at the point 2, thereby efiectively disabling the latter path at thatpoint. Thus, any echoes of the incoming westto-east signals appearing inthe input of the path WA, dueto improper unbalance of the hybrid coil Hand associated balancing network N, or any subsequentlyreceivedeast-to-west signals received over the circuit TC, will beprevented from passing out over the path WA to the west station or fromentering the receiving switching control RS to cause false operation ofecho suppressor ES. Also, in the ordinary operation of echo suppressorES, when the transmitting control TS is not operated'byincomingwest-to-east signals, and east-to-west signals received over the circuitTC are impressed on the outgoing path WA th'rough hybrid coil H, aportion of these signals diverted into the receiving switching controlRS, which also may include an amplifier-rectifier (not shown) will causeits operation to operate switching relay RE to short-circuit theincoming path EA at the point i, to suppress any subsequently receivcedwest-to-east signals or echoes. In a modified echo suppressor, the relayRw under control of transmitting switching control TS would operate toclose the contacts at point 2 to insert loss in the path WA, as shown,but RE under control of receiving switching control RS would operate todisable the transmitting switching control TS of the echo suppressor toavoid false operation, etc. In such a case, a similar echo suppressordevice at the west terminal of the system (not shown) would be arrangedto operate in similar manner to block the outgoing path EA in responseto incoming signals in the incoming path WA. In the suppressor as shownon Fig. 1, no echo suppressor need be used at the distant end of thecircuit. I

The control circuits for accomplishing the objects of the invention,illustrated in Fig. 1, include one control branch 3 having its inputbridged across the incoming path EA at a point in front of amplifier As,and a control branch 4 having its input bridged across the input of theoutgoing path WA at a point in front of amplifier Aw. The control branch3 includes the amplifierdetector 5 and connected in parallel across itsoutput the left-hand operating winding of differential relay 6 and theoperating winding of relay l. The control branch 4 includes theamplifier-detector 8 and connected across its output the right-handwinding of the diiferential relay 6, poled to oppose the left-handwinding of that relay. The differential relay 6 and relay 1 control theoperation of the relays 9 and I0 and the associated circuits in themanner to be described in the following complete description of theoperation of the circuits of Fig. 1.

With no speech current being received over the incoming path EA and nospeech currents being transmitted over the outgoing path WA, thecircuits described above are in the condition shown in the drawing, withthe differential relay 6 deenergized so that its armature is in theneutral position, the relays land 9 deenergized and the cuits TS and RSeffective for echo-suppressor operation.

Now, let it be assumed that speech currents from a west subscriber arebeing received over the incoming transmission path EA at the eastterminal of the system, and at that time no speech currents from an eastsubscriber associated with the circuit TC are present in the outgoingpath WA at the east terminal. The incoming speech waves in path EA areamplified in the amplifier AE and, the echo suppressor relay RE being inits normally unenergized condition so that the path EA is unblocked atthe point I, the amplified voice waves will pass over the output portionof the path EA and through the hybrid coil H to thetwo-way circuit TCover which they will be transmitted to the east subscriber. A portion ofwests speech waves will be diverted into the transmitting control TS ofecho suppressor ES and will operate relay Rw to block WA at the point 2.

Another portion of wests speech waves passing over the path EA will bediverted into the control branch 3 of the differential control circuit,will be amplified and detected by the amplifier-detector 5 therein, andwill be supplied as energizing current to the operating winding of relay1 to operate that relay, and to the lefthand operating winding of thedifferential relay 6 tending to operate that relay upwards. The echoesof wests signals produced in the outgoing path WA due to improperbalance of the circuit TC by the balancing network N will be transmittedover that path through amplifier Aw towards the west terminal.

A portion of the echo currents in the input of amplifier Aw will bediverted into the control branch 4 of the differential control circuit,will be amplified and detected in amplifier-detector 8 and supplied asenergizing current to the righthand winding of differential relay 6tending to operate that relay downward. The sensitivity ofamplifier-detector 5 in control branch 3 and that of amplifier-detector8 in control branch 4 are relatively adjusted so that the difierentialrelay 6 operates downward to close its armature and lower contact whenthe echo in the path WA is greater than desired compared to the directsignal transmission in the path EA, and operates upwards to close itsarmature and upper contact when the echo in the path WA is weak enoughcompared to the direct transmission in the path EA to be negligible.When the differential relay 6 is operated downwards, the relay 9 remainsreleased (as in its normal condition) and thus the relay l0 controlledthereby remains in the operated condition with its upper and lowercontacts closed to maintain the transmitting switching control TS andthe receiving control RS of echo suppressor ES operative so that theecho suppressor will continue to function to maintain the outgoing pathWA disabled at the point 2 so as to suppress the echoes of wests signalsat that point in the manner described above. However, when thedifferential relay 6 is operated upward indicating that the echo in thepath WA is negligibly weak compared to the direct signal transmission inthe path EA, relay 9 tends to operate from battery I I through groundapplied through the upper contacts of differential relay 6, but, due tothe effect of condenser l2 in shunt with its winding and the resistanceI3 in series therewith of properly selected values, relay 9 is slow tooperate, requiring either a continuous closure of the upper contacts ofdifferential relay 6 for a given time interval, or a considerable numberof shorter closures of these upper contacts to charge up the condenser[2 before relay 9 operates. This is for the purpose of insuring that thereturn loss over the frequency band of interest is adequate, as a fastoperating relay 9 might be improperly operated by a high return loss inone frequency range when a sound contained mostly those frequencies.

If at any time during this process, the differential relay 5 releasesand stays neutral, nothing happens except that in time the charge oncondenser I! would drain away through relay 9. However, if thedifferential relay 6 opcrates downwards while relay '1 is operated,condenser l2 will be shorted by a circuit extending through the madecontacts of relay 1 and the made lower contacts of differential relay 6,and the charging process must begin over again before relay 9 can beoperated. When relay 9 does operate, removing ground from the normallyenand. RS forecho suppressor. ES and thus disabling the suppressor sothat it is efiectively re- 'moved from the system. Relay 9 will lock upthrough its operated front contact and ground. Relay Swill then remainoperated to hold relay l deenergized and. thus. the echo suppressor ESinefiective untilv such time as. the differential relay 6 again becomesoperated downward in response to a greater echo in the path WA thandesired compared to the direct transmission, and relay 1 is operated.simultaneously in response to the speech waves receivedfrom the path:EA. When. this happens, the winding. of relay 9- will be shortcircuitedthrough the contacts of relay 1 and the lower contacts of. relay 6 andrelay 9 will. therefore release to return relay I0: to. the normalenergized condition whereupon: the echo suppressor ES again becomesoperative.

The purpose of relay 1 is to prevent the near.-

end subscriber at the local toll terminal from restoring the echosuppressor to the circuit when he talks. For example, if the contacts ofrelay 1 were permanently closed when the subscriber at the near-endtalks, differential relay 6 would be operated downward causing relay 8to be. released and relay [8- operated, thereby restoring the echosuppressor ES to the circuit while there is no need for it,v Withrelay'l released, however, as it'would be with no speech coming in fromthe distant end of the four-Wire circuit, the. operation of thedifferential relay d downward will not cause the release of. relay 9with the result that. the echo suppressor ES will remain inoperative.

This raises the question as to What happens when the. west subscriber atthe distant end of the four-wire circuit and the" local subscriberassociated with the circuit TC start talking at about the same time. Inthis case, the voice currents of the east subscriber would maintaincontrol of the difierential relay 6- with the result that the echo.suppressor ES will. be. maintained operative to disable the incomingpath EA until thusrequiringconsiderable overlap in simultane- 1 ousspeech before the echo-suppressor restored. If this were not sufficient,it might be desirable to provide another relay operating from the out.-put of the receiving amplifier-detector 8 with such low sensitivity thatitnever would be operated inresponse. to echoes but would operate inresponse to loud. near-end speech, and arranged for the release of thelatter relay to closea break in wire M connecting the armature of relay6 to the Winding of relay 9. Then Withloud. speech from the nearend,.the echo suppressor ES would not be restored by double talking,since the open contact on the added relay would prevent relay from beingreleased.

. In the above discussion, it will be understood that when the echosuppressor is stated to be operative and later inoperative, this mightmean present on the circuit and later actually removed from the circuitand available for use on other circuits. Also, the two conditions mightrefer to difierent echo suppressor settings, say high and low zero levelsensitivitiesxor two different values of suppression loss. Amodification within the scope of the invention would be to provide twoor more control circuits such as illustrated inv Fig. 1. associated withthe incoming and outgoing paths. EA and. WA in similar manner as thecontrol circuit illustrated, which operate to reduce the sensitivity ofthe echo suppressor ES in two or more steps, to obtain as little echosuppressor operation as possible.

Other modifications of the control circuit of Fig. 1 might be desirablein the interest of economy. For example, relay '1 might well becontrolled fromthe amplifier-detector in the transmitting echosuppressor control TS instead of from the auxiliary amplifier-detector 5as illustrated, and. the two amplifier-detectors 5 and 8 might be partlyor wholly made up of the amplifier-detectors normally provided in thetransmitting andv receiving switching controls TS and RS of theechosuppressor ES. In addition, the differential relay 6 might comprisea differential thermistor Wheatstone bridge l6 and an associated relay22, such as illustrated in Fig. 2, or other differential. means.

In: thesystemof Fig. 2, the diiierential bridge It includes thethermistors l1 and I8. in two arms of the bridge and the equal resistorsl9 and 20 in the other two arms of. the bridge. A source. of current 2!,which may be. a. direct current battery, is connected across onediagonal of the bridge. and the operating winding of. the relay 22. isconnected across 'the other bridge diagonal. The heater of thethermistor If! in one arm of the bridge. is connected to the. output of.the amplifier-detector 5 in the transmitting control branch 3. across. ashunt condenser pro videdto suppress the alternating component of thedetector output current from the heater element, and the heater elementof the-thermistor,

l8- in the opposing arm of the bridge is. con.- nected across anothercondenser provided for a similar purpose to the output of theamplifierdetector 8a in the receiving control branch 5. The relays. E,9i and it, and: their associated resistance and condenser elements arearranged inthe same manner as the similarly numbered elemerits-in thesystem of Fig; 1 and provide. the same functions.

When voice currents from the path EA are impressed on theamplifier-detector 5 of control circuit t, and; its output suppliesheating current to. the heater of thermistor ll, which is relatively.greater than that applied to the heater of thermistor ill by theamplifier-detector 8 in the receiving. control. branch 4 in response tothe signal echoes in that branch, the resistance value of thermistor llbecomes comparatively smaller than the: resistance value of thermistor 8with the result that the Wheatstone bridge it. formed by the thermistorelements 1! and i8 and. the equal. resistors l9 and 23, is unbalanced,and the unbalance current of the bridge flows through theoperating'winding of relay 22. Relay 22.is normally biased by thecurrent supplied to its biasin winding from battery 23 so that theunbalance current flowing in the operating winding of the relay tends tooperate its armature upwards towards the grounded upper contact. Whenthis unbalance current is greater than a predetermined amount indicatingthat the signal echoes in the path WA are negligibly small relative tothe direct signal transmission in the path EA, the relay 22 will beoperated to the grounded upper contact causing relay 9 to be operatedand thus relay H] to be released to disable the echo suppressor ES inthe same manner as described for the system of Fig. 1.

On the other hand, when the current through the heater for thermistor I8is relatively greater than that through the heater of thermistor llindicating that the amount of signal echo in the path WA is too greatwith respect to the direct signal transmission in the path EA, theWheatstone bridge IE will be unbalanced in the opposite direction andthe unbalance current will flow in the opposite direction in theoperating winding of relay 22, operating the armature of that relay toits lower contact. Then the winding of relay 9 will be shorted throughthe made lower contact of relay 2| and the made contact of relay 1 whichhas been operated by the output currents of amplifier-detector 5. Relay9 will therefore release to reenergize the winding of relay I0 and thelatter relay will reoperate to restore the echo suppressorES to thecircuit in the manner which has been previously described in connectionwith Fig. l.

Other modifications of the circuits illustrated and described which arewithin the spirit and scope of the invention will be apparent to personsskilled in the art.

What is claimed is:

1. In combination in a two-way signaling system, a four-wire circuit fortransmitting signals in opposite directions, a two-way signaltransmission medium coupled to one terminal of said fourwire circuit, anecho suppressor device responsive to signal transmission in the side ofsaid four-wire circuit transmitting to said medium to insert an echosuppression loss in the other side of said four-wire circuit and meanscontrolled by the transmission in both sides of said four-Wire circuitfor effectively disabling said echo suppressor device when the level ofsignal echoes in said other side of said four-wire circuit is less thana predetermined value and for rendering said echo suppressor deviceoperative again when the level of the signal echoes in said other sideof said four-wire circuit is above the predetermined value.

2. The combination of claim 1, in which the last-mentioned meanscomprising a differential circuit for continuously comparing the signalsin said one side with the echoes of said signals in said other side ofsaid four-wire circuit, means controlled by said differential circuitoperating only when the ratio of the echo level in said other side tothe signal level in said one side of said four-wire circuit is withinpredetermined limits considered tolerable, to disable said echosuppressor device.

3. The combination of claim 1, in which the last-mentioned meanscomprises differential means for continuously comparing the level of thesignals in said one side of said four-wire circuit at a point in frontof the input to said echo suppressor device with the level of the echoesdu to said signals at a point in said other side of said four-wirecircuit in front of the suppression point, and means controlled by saiddifferential means and operating only when the ratio of the echo levelto the signal level is within predetermined tolerable limits, to disablesaid echo suppressor device.

4. In combination in a two-way signal transmission system, a two-wayline, a four-wire terminating circuit operating to transmit signals to,and to receive signals from said line, one switching circuit connectedto the transmitting side of said four-wire circuit, responsive to signaltransmission therein to disable the receiving side of said circuit tosuppress echoes therein, another switching circuit connected to saidreceiving side of said four-wire circuit at a point beyond the disablingpoint therein, operating when said receiving side is operative, inresponse to signal transmission therein to disable the transmitting sideat a point beyond the point of connection of said one switching circuitthereto, a differential circuit continuously operating to compare thelevel of signals in said transmitting side at a point in front of thedisabling point therein with the level of the signal echoes in saidreceiving side of said four-wire circuit at a point in front of thedisabling point therein, relay means controlled by said differentialcircuit operating only when the echo level is relatively less withrespect to the signal level than a predetermined value considered astolerable, to disable both switching circuits, and means to preventwhile signal transmission in said transmitting side continues, the twoswitching circuits from being restored to normal in response to signalssubsequently received by said four-wire circuit from said two-way line.

5. The system of claim 4, in which said differential circuit comprises adifferential relay hav ing two opposing windings respectively energizedby transmission in the two sides of said four-wire circuit, operating inone direction when the signal echo in said other side compared to thedirect signal transmission in said one side of said fourwire circuit isgreater than considered tolerable, and in the opposite direction whenthe echo is sufiiciently weak compared to the direct signal transmissionto be within the tolerable limits, and a relay circuit operating onlywhen said differential relay operates in said opposite direction, todisable both said switching circuits.

6. The system of claim 4, in which said difierential circuit comprises aWheatstone bridge circuit including two opposing arms the resistances ofwhich are respectively varied in accordance with the level of thetransmission in th respective sides of said four-wire circuit, and saidrelay means is responsive only to an unbalance condition of said bridgecorresponding to a ratio of the transmission levels in the two sides ofsaid fourwire circuit obtained when the signal echo in said other sideof said circuit is within the tolerable limits, to disable the twoswitching circuits.

LEONARD G. ABRAHAM.

